John McDonnell has 'fallen into Osborne's trap', says Caroline Lucas

John McDonnell has fallen into George Osborne’s trap by pledging to vote in favour of the chancellor’s proposed fiscal charter, Caroline Lucas has said.

Speaking to the Guardian at her party’s autumn conference in Bournemouth on Saturday, the Green party MP said she understood the pressure that the Labour shadow chancellor was under but that his decision reinforced the austerity narrative which Osborne had been “so clever” in propagating.

“I was glad to see in the small print that [McDonnell] made a distinction between the current account and investment and I’m glad to see that he’s still talking about borrowing to invest in jobs,” said Lucas.

“What worries me about what he said ... is that it does rather unhelpfully reinforce the framing that Osborne has been so clever at portraying, which is that the deficit is so important that it has to be the number one priority and therefore we have to put every other policy at risk in order to get the deficit down to a pretty arbitrary timetable.”

Lucas added: “I think [McDonnell] has fallen into a trap that Osborne has set him. But I say that more in sorrow than anger. I recognise the pressure that he’s under.”

The MP for Brighton Pavilion is the Green party’s only MP, having increased her majority in the general election from 1,252 to just under 8,000. The party increased its overall share of the vote fourfold in May, winning 1.1m votes and 4% of the electorate – their best election result ever. However, Corbyn’s victory in his party’s leadership election has thrown the Greens’ position as the more leftwing alternative to Labour into question, with them losing 1,000 members since August.

Lucas will give a speech to the party’s biggest ever conference on Sunday, looking ahead to the Paris climate talks at the end of November. “For more than 20 years, governments have been meeting at global conferences to talk endlessly about the crisis, yet greenhouse gas emissions have continued to rise,” she is expected to say, appealing to her party’s members not to lose hope.

“In 10 weeks’ time, Paris will also be home to the world’s largest non-violent direct action civil disobedience,” Lucas will say. “It will be home to a mass mobilisation from global movements that aim to leave political leaders no other choice than to change everything.”

Speaking ahead of her speech, she said that she hoped the Green party could work with the Labour party in the runup to the talks to put pressure on the government. “I think there is a bit of a battle going on certainly inside the Labour party right now ... but I hope very much that Jeremy Corbyn’s view will prevail, which, as I understand it, would be a much stronger message against extraction of fossil fuels.”

“I’m hoping that a progressive Labour party under Jeremy Corbyn might be a good partner for us in making that case even more loudly to the government,” she added.

“If we are serious about getting progressive ideas in government, we need to join forces to defeat the Tories,” said Lucas. “I think Labour is not going to win the next election. They’d need to win over 100 seats, they’d need to take back Scotland ... Perhaps there might be some arrangement whereby local parties can discuss whether they might want to have some kind of alliances running up to the election in 2020, because the bottom line is if we keep fighting one another then the only beneficiaries of that are the Conservatives.”